Inboard Engine Parts

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Surface Gap Spark Plugs?

December 21st, 2010, 12:19 PM

I've always used the NGK B8HS10 spark plugs in my 90HP Yamaha with no problems, but read occasionally about these surface gap plugs without the "hook-type" ground electrode, and just wondering what is the advantages or disadvantages of these plugs? I believe some airplanes use surface gap plugs also. Anybody know? Thanks!

Comment

If one does a lot of high speed running (as in long periods of WOT) then a surface gap plug seems to have an advantage. I would not consider switching from a conventional to surface gap plug unless there was some specific set of operating conditions you experience that would give cause for this consideration. I have in the past switched from surface gap to conventional plugs for specific reasons. HOWEVER -- there generally is no direct cross reference between surface gap and conventional plugs so be darned sure you understand heat range issues before you go either way with this. Powerheads are much more expensive to replace than changing plug types "just because!!!"

Comment

Thanks, I was just curious. I've seen some other plugs with multiple electrodes with as many as 6 points that claim to "fire hotter", but that makes no sense, since the spark is naturally going to follow the one path of least resistance, which would be whichever electrode is closest to the center, seems like just a sales gimmick to me. I'll just stick with my NGK B8HS-10's.

Comment

Bob: Once you understand the engineering behind sparkplugs, then it all becomes simple.

Sparkplugs are designed in Heat Ranges for different applications. The heat range indicates basically how hot the plug will run in the engine.

The center electrode and porcelain insulator of the sparkplug are exposed to combustion temperatures and with each cycle they heat and cool.

The amount of center electrode and porcelain exposed to combustion temperature determines how hot they will run. Surface gap plugs have almost no electrode and insulator exposed to temperature AND the steel shell of the plug body provides a short path for cooling through the cylinder head. These plugs run COLD. In a 2 cycle engine where there is one power pulse per revolution, (twice that of a 4 cycle) the plug is exposed to MORE heat and less cooling time, so frequently, these plugs are specified for outboard engines.

On the positive side, these plugs protect the engine from damage. Yes, I have seen engines designed for surface gap plugs with holes melted in the piston crowns as a result of using bent electrode plugs --Evinrude QL77 type--(which run much hotter). On the negative side, these surface gap plugs require the extremely high voltage of an electronic ignition. Most will not fire at low speeds with a magneto ignition or a regular points and battery ignition. The do tend to foul and misfire with prolonged trolling or low speed operation.

Many people say that there is no danger in substituting bent electrode plugs and the idle substantially improves, BUT: Do you think it was an accident that Mercury engineers specify the L76V surface gap plug for their engines?

As far as multiple electrode plugs like Splitfire or Bosch, all a bunch of marketing hype. B.S. and hogwash of the first degree.

Electricity takes the shortest path OR the path of least resistance. With multiple electrode plugs like the 4 electrode Bosch, the spark will preferentially jump to only one electrode until that electrode has higher resistance than another one. There will NOT be multiple sparks at one time. (Note that MSD ignition modules DO generate multiple sparks at the same ignition cycle but I do not have an opinion on any benefits outside of racing)

Hotter spark? Phooey! Blue-white is blue-white. What is a couple of hundred degrees difference (if any at all) in a couple of thousand when there is more than sufficient temperature to ignite the air/fuel mixture? Again, marketing hype! Why use a propane torch to ignite a cigarette when a match will do it just as well?

For your engine, use the BRAND of sparkplug that you like in the STYLE specified by the manufacturer.

Comment

My 1982 Merc 50 uses surface gap plugs. It idles very smoothly at 1,000 rpm and there isn't an idle adjustment for this engine. When I got the boat I was getting pissed because the plugs were always black and horribly dirty no matter how often I cleaned them and tried to avoid long idles. I did some checking and found it had the wrong heat range plugs in it. I removed the L76V plugs and installed the correct L78V plugs. (Actually I bought the NGK's) Then I ran it two more days back and forth to the local sandbar party before checking it again. Surprisingly the plugs looked like I had just taken them out of the box, very clean with a slight brownish tint beginning to form.

On a side note, I'm personally a big fan of NGK brand plugs and for some unexplainable reason I never use Champions. I went to West Marine, and found the Champion L78V plugs for $6.99 each.... then cross referenced them to the NGK Part# BUHW-2 which were only $3.99 each. I saved $12 and bought the NGK's.

Now here's something a little more interesting- The original engine owner's manual for my engine lists the NGK spark plug part#BUHW-2 as OEM required plugs. The aftermarket Seloc maintenance manual specifies the Champion plugs and doesn't even list the correct NGK OEM plug part# anywhere in the manual..... Hmm, did Champion sponsor this maintenance manual?

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My 84 merc 90hp inline6 and my 81 merc 175hp v6 both came with the surface gap plugs. They work well so I never saw any reason to chance tinkering with trying different styles of plugs. Use what the manufacturer specifies.

Comment

Silvertip, you should elaborate Tell him why you don't change horses in the middle of the stream. Humm, where is my copy of the two stroke tuners hand book. LOL..

Seriously though, what Silvertip is telling you is that your motor will run better and last longer if you use the factory recommended plug. This is a pretty well known fact around the two stroke world. If the engine was designed for surface gap plugs then use them, if not don't. It's all about the design.

Comment

Own a copy of two stroke tunners book too . Used on 120 hp chrylser and loved . first of year fire up could choke with hand gas everywhere would fire up not fouling , same plugs for 10 plus boating seasons , could troll 20-30 min untill hot buzzer came on reved up in neutral til lit went off and trolled again . Found them in my daughters mazda RX7 only 90 degree slots to let excess fuel drain off but this engin had cold start glycol inject and if you didn:t let warm up and shut off and restarted it would run like skeeter fogger or flood and the are tough to recover from most people gave up sold cars but the plugs defently have applacations and advantages.
Will be trying in my McCullough 100ci 4cyl 2 stroke drone motor also